In recent years, since there are problems of a depletion of fossil fuels such as petroleum and global warming caused by carbon dioxide emissions, popularization of new systems as an alternative to a conventional power generation system has accelerated. As one of the new systems, fuel cells have received attention, since they are very practical as dispersed power sources and power sources of vehicles. There are several types of fuel cells, especially, a polymer electrolyte fuel cell (PEFC) and a solid oxide fuel cell (SOFC) have high energy efficiency, and are expected to increase in popularity in the future.
The fuel cell is an apparatus for generating power by performing a reaction process that is opposite to that performed during electrolysis of water, and in which hydrogen is used. Hydrogen is produced by reforming a hydrocarbon fuel such as a city gas (LNG), methane, natural gas, propane, lamp oil, and gasoline in the presence of a catalyst. Especially, the fuel cell using the city gas as a raw fuel have advantages of being able to produce hydrogen in an area improved utility piping.
A fuel reforming reactor is commonly operated at a high temperature in a range of 200° C. to 900° C., in order to secure the amount of heat required for the hydrogen reforming reaction. Further, the fuel reforming reactor is subjected to an oxidizing atmosphere including a lot of water vapor, carbon dioxide, carbon monoxide, and the like, in the high temperature condition, and a heating-cooling cycle is repeated by a start-and-stop control depending on hydrogen demand. Austenitic stainless steels represented by SUS310S (25Cr-20Ni) has been used as a practical material having sufficient durability under the above hard conditions. A cost reduction of the fuel cell system is required to expand the popularization of the fuel cell system, and it is necessary to reduce the alloy cost according to optimizations of raw materials.
Further, in the SOFC system, it is necessary to prevent a ceramic electrode from being poisoned from Cr vaporization at the SOFC operating temperature, while the stainless steel containing high levels of Cr is used in the SOFC system. CrO3 (g) having a high vapor pressure is generated from Cr2O3 (s) formed on a surface of the stainless steel, based on reactions represented by following formulas (1) and (2); the ceramic electrode is subjected to vapor-phase diffusion of CrO3 (g) and Cr2O3 (s) is deposited onto the ceramic electrode, thereby the ceramic electrode is poisoned from the Cr vaporization. e− is transferred to the ceramic electrode under ordinary circumstances, but when e− is used as shown in the formula (2), Cr2O3 (s) is deposited onto the ceramic electrode, and as a result, the internal resistance of the fuel cell increases and the power generation efficiency decreases.½Cr2O3(s)+¾O2(g)=CrO3(g)  (1)CrO3(g)+3e−=½Cr2O3(s)+ 3/2O2−  (2)
Note: (s): solid, (g): gas, and e−: electron.
From the above-described circumstances, in order to ensure good oxidation resistance and suppress Cr vaporization, the use of a ferritic stainless steel containing Al is recommended. In Patent Document 1, a ferritic stainless steel for a reformer of reforming petroleum fuel is disclosed, and the ferritic stainless steel is prepared by including Cr: 8 to 35%, C: 0.03% or less, N: 0.03% or less, and Mn: 1.5% or less; Si: 0.8 to 2.5% and/or Al 0.6 to 6.0%, and one or two or more elements selected from a group consisting of Nb: 0.05 to 0.80%, Ti: 0.03 to 0.50%, Mo: 0.1 to 4%, Cu:0.1 to 4%, wherein the total amount of Si and Al is 1.5% or more. In the above stainless steel, the increase in the amount of oxidation during heating at 900° C. and cooling in atmosphere of 50 vol % of H2O+20 vol % of CO2 is small.
The ferritic stainless steel used in a reforming reactor for reforming alcohol fuel is disclosed in Patent Document 2, and the ferritic stainless steel includes Cr: 8 to 25%, C: 0.03% or less, N: 0.03% or less, Si: 0.1 to 2.5%, Mn: 1.5% or less, Al: 0.1 to 4%, and one or two or more elements selected from a group consisting of Nb: 0.05 to 0.80%, Ti: 0.03 to 0.5%, Mo: 0.1 to 4%, and Cu: 0.1 to 4%. In the above stainless steel, the increase in the amount of oxidization is 2.0 mg/cm2 or less, after heating at 600° C. and cooling were cycled repeatedly in 500 times in an atmosphere of 50 vol % of H2O+20 vol % of CO2.
A ferritic stainless steel which is preferable for use in a power-generating system is disclosed in Patent Document 3, the ferritic stainless steel includes Cr: 11 to 22%, C: 0.03% or less, N: 0.03% or less, Si: 2% or less, Mn: 1.5% or less, and Al: 1 to 6%, and a total amount of Cr, Si, and Al satisfies Cr+5Si+6Al≥30. The above stainless steel has good oxidation resistance at a temperature of 700° C. or 800° C. and an atmosphere of 50 vol % of H2O (and a balance of air), and an Al oxidation layer containing 5 mass % or less of Cr is formed thereon, and an Al absence layer is formed on an inside surface of the Al oxidation layer, thereby preventing Cr vaporization.
A ferritic stainless steel which is preferable for use in a high temperature reforming apparatus of a fuel cell is disclosed in Patent Document 4, the ferritic stainless steel includes Cr: 11 to 21%, C: 0.03% or less, N: 0.03% or less, Si: 3% or less, Mn: 1.0% or less, Al: 6% or less, Cu: 0.01 to 0.5%, Mo: 0.01 to 0.5%, Nb: 0.1% or less, Ti: 0.005 to 0.5%, Sn: 0.001 to 0.1%, O: 0.002% or less, H: 0.00005% or less, and Pb: 0.01% or less. The above stainless steel has a good oxidation resistance at a temperature of 1200° C. in an atmosphere of 10 vol % of H2O (and a balance of air).
A ferritic stainless steel containing Al which is used for a fuel cell is disclosed in Patent Document 5, the e ferritic stainless steel includes Cr: 13 to 20%, C: less than 0.02%, N: 0.02% or less, Si: more than 0.15 to 0.7%, Mn: 0.3% or less, Al: 1.5 to 6%, Ti: 0.03 to 0.5%, Nb: 0.6% or less, and amounts of Ti solid solution and Nb solid solution are controlled to improve oxidation resistance and creep fracture lifetime. The above stainless steel has good oxidation resistance in an accelerated oxidation test at a temperature of 1050° C. in air.
The ferritic stainless steel disclosed in Patent Documents 1 and 2 indicate improvements of the oxidation resistance under the atmosphere of 50 vol % of H2O+20 vol % of CO2. In the former, the technological thought is that properties of a Cr oxide film are improved by a combined addition of Si+Al>1.8% while in the latter, the technological thought is that the properties of an Al oxide film and a Cr oxide film are improved by a combined addition of Si+Al. However, when the Cr oxide film is formed on the ferritic stainless steel, it is difficult to avoid vaporization Cr as described above in formulas (1) and (2).
The ferritic stainless steel disclosed in Patent Document 3 indicates improvements of the oxidation resistance under atmosphere of 50 vol % of H2O (and a balance of air), the technological thought is that the Al oxidation layer containing 5 mass % or less of Cr is formed by a combined addition of Si+Al, and an Al absence layer is formed in an inside surface of the Al oxidation layer, thereby preventing Cr vaporization. Preliminary oxidation conditions for forming the Al oxidation layer is at a temperature of 800 to 1100° C. in an atmosphere consisting of a mixture of air and carbon dioxide having a dew point of 20° C., in 10 minutes or less.
The ferritic stainless steel disclosed in Patent Document 4 is limited to a 18Cr-1.9 to 3.3Al in which B is not added and Sn is added as an essential element. In the ferritic stainless steel disclosed in Patent Document 5, the oxidation resistance of the Al oxidation layer is improved by reducing generation of Ti oxide during the accelerated oxidation at a temperature of 1050° C. by reducing the amount of Ti solid solution, and the creep fracture lifetime is improved by ensuring the adequate amount of Nb solid solution by adding Nb. That is, the generation of the Ti oxide is reduced under the above technological thought.